This invention pertains to an aqueous inkjet ink with a certain additive and to a method of printing with this ink whereby optical density (OD) of a printed image is increased. The additive is particularly advantageous for increasing the OD of inks with a self-dispersing pigment colorant.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Both dyes and pigments have been used as colorants for inkjet inks. Inks comprising pigments dispersed in aqueous media are considered advantageous because printed images tend to be waterfast and lightfast.
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Traditionally, pigments have been stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. More recently though, so-called “self-dispersible” or “self-dispersing” pigments (hereafter “SDP”) have been developed. As the name would imply, SDPs are dispersible in water without dispersants.
SDPs are often advantageous over traditional dispersant stabilized pigments from the standpoint of greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink.
Still, there is a need for pigment inks with improved coloristic properties. Compositions and methods that provide increased optical density and chroma are desirable.